After the import process builds the invoices, the invoices can be viewed, modified, and validated on the Manage Invoices page.
References:https://fusionhelp.oracle.com/helpPortal/topic/TopicId_P_A149B7BD08207B6DE040D30A68816335

A: Account Rules by Segment
Define segment rules to derive a specific segment of the general ledger account. For example, a particular segment like the company segment can be determined from the distribution account. Another segment can be determined with the use of a constant value. Creating the account one segment at a time offers greater flexibility, but also requires more setup.
C: You can use a Distribution Set to automatically enter distributions foran invoice when you are not matching it to a purchase order. For example, you can create for an advertising supplier a Distribution Set that allocates advertising expense on an invoice to four advertising departments.
References:https://docs.oracle.com/cd/A60725_05/html/comnls/us/ap/distsets.htm

Typically General Ledger users are secured by data access sets. Data access sets use primary balancing segment values to secure access to ledgers, ledger sets, and portions of ledgers. Payables users are secured
by business units.
For the Payables to General Ledger Reconciliation report:
General Ledger users can see general ledger data for the balancing segment values in their data access set. General Ledger users can also see the Payables or Oracle Fusion Subledger Accounting data for all
business units linked to the ledger.
Payables users can see the Payables and Subledger Accounting data for business units in their security definition. Payables users can also see general ledger data for all balancing segment values in the ledger.